1. Field of the Invention
The present invention relates generally to electrooptical measurement of aerosols as a means for determining the mass concentration of the particles or droplets contained therein, and more particularly, pertains to an electrooptical system and technique for direct quantitative measurement of the mass concentration of the constituent particles or droplets of a given monodisperse aerosolized medium.
2. Description of the Prior Art
Heretofore, various techniques have been employed in endeavors to accurately and precisely determine various characteristics and properties of aerosolized particles. Many of these prior techniques utilize devices and equipment necessitating the transfer or extraction of a representative sample of the aerosol which is to be analyzed, or measured, in order to determine the properties and characteristics of the particle or droplet constituents of the aerosol under investigation. Obviously, the ability to obtain a truly representative sample is dependent to a great extent upon the manner in which the sample is obtained. No less importantly, the sampling technique and equipment, or device, must be designed in such manner that it provides the capability of extracting a truly representative sample of the aerosol to be subjected to study or at least be capable of providing a sample which is representative within a known margin of error.
One commonly followed approach to the problem of determining the accuracy, or efficiency, of the sampling technique, or device, is to test the sampling capability thereof by the expedient of comparing its sampling results against the sampling results obtained with a standard sampler tube which has been precalibrated, or pretested, for sampling accuracy and, hence, possesses a known efficiency with respect to the aerosol properties and characteristics for the procedures used for such precalibration. If the precalibration was obtained by comparison with another precalibrated standard sampler, as discussed above, which in turn was similarly precalibrated, etc., the degree of the accuracy of the standardization technique tends to become increasingly suspect as each new generation of standard samplers evolves. Alternatively, a somewhat better approach for determining the accuracy, or efficiency, of the sampling device is to utilize it in conjunction with a test, or standard, aerosol possessing known properties or characteristics, such as, for example, known composition, mass, or number concentration, and particle size distribution. In accordance with this latter approach, a quantity, or aliquot portion, of the test aerosol ordinarily is aspirated at a known flow rate for a known period of time into the aerosol sampler from a chamber containing the test aerosol. Thereafter, the mass of the aerosol collected within the sampler is gravimetrically determined in conventional manner. The particle mass of the aerosol collected by the sampler is then compared directly with the known, mass concentration of the test aerosol contained within the aerosol test chamber to thereby determine the accuracy, or efficiency, of the sampler. However, while this latter approach provides a more reliably accurate method of original standardization, or precalibration, of an aerosol sampler, it is dependent upon a comparative assessment of the results obtained by the sampler and the results ordinarily obtained by difficult and extremely complex Mie-theory calculations, aerosol model-studies, and the like from which the needed information with respect to particle mass, mass concentration, particle size-distribution and the like are derived. Obviously, such calculations and studies are extremely costly and time-consuming. Hence, the art of accurate, aerosol sampling is in great need of a technique and apparatus capable of providing extremely accurate monitoring of the physical characteristics of aerosolized particles and of providing a highly accurate means of determining the sampling efficiencies of aerosol samplers generally or for standardization and precalibration purposes.
Since the true accuracy, or efficiency, of the sampler is based upon the accuracy of the known values attributed to the test, or standard, aerosol from which the sample was taken, it is, of course, exceedingly important that such properties as, for example, the particle size, mass concentration and the like, of the test aerosol be monitored as closely as possible for knowledge of any changing conditions. Such close monitoring, however, is very difficult and presents an extremely complicated and challenging task, since the establishment and maintenance of a test aerosol possessing a high degree of constancy in its properties and characteristics is an extremely complex problem. Customarily and according to past practices, the test aerosol is periodically sampled to ascertain if, and in what manner, the properties and characteristics thereof have undergone change as a result of an elapse of time during which particle agglomeration and settling commonly can and do tend to occur. This problem is frequently further aggravated by the fact that the sampling of the test aerosol involves the withdrawal of a representative sample which in and of itself constitutes a source of disturbance and diminishment of the chamber-contained, test aerosol. Moreover, during the time required to complete an accurate analysis of each withdrawn sample of test aerosol, which ordinarily requires at least several minutes duration, the properties and characteristics of the aerosol within the test chamber remain subject to continued or further alteration. In view of these and other problems associated with past aerosol sampling procedures, a technique for more precisely and more rapidly monitoring the properties and characteristics of a test aerosol of the foregoing type is a matter of significant importance and concern.